fix: metadata validation causes AttributeError for patch requests

[matthewelwell]

Changes

Fixes AttributeError raised by metadata validation logic when using patch request on environments viewset.

How did you test this code?

Added a new unit test to verify that the issue is resolved.

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Name	Status	Preview	Comments	Updated (UTC)
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[github-actions]

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[codecov-commenter]

Codecov Report

Patch coverage: 94.11% and no project coverage change.

Comparison is base (cb42f9d) 95.43% compared to head (bd11011) 95.43%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2614   +/-   ##
=======================================
  Coverage   95.43%   95.43%           
=======================================
  Files         975      975           
  Lines       27245    27252    +7     
=======================================
+ Hits        26002    26009    +7     
  Misses       1243     1243           

Files Changed	Coverage Δ
api/environments/serializers.py	95.52% <85.71%> (-1.41%)	⬇️
api/environments/tests/test_views.py	100.00% <100.00%> (ø)
api/metadata/serializers.py	91.89% <100.00%> (+1.35%)	⬆️

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